Conserved versus innovative features in animal body organization.

The origin of evolutionary novelties is a central topic in evolutionary developmental biology (evo-devo) studies. In any new feature, there is a conserved component that is either structural or related to the underlying genetic control, but it is not always obvious what is really new and what is conserved. Nevertheless, disentangling this blending of old and new features is basic to understanding mechanisms of evolutionary change. The origin of arthropod appendages illustrates the complexity in tracing the origin of evolutionary novelties. At the base of the lineage, the main body axis was already segmented and antero-posteriorly patterned, and the genetic tool kit required to form lateral outgrowths was already available. The novelty was possibly the developmental decision to "read" the available axial information and to exploit it for differentiating segmentally patterned and axially segmented appendages. Some important novelties bridge the gap between what have been traditionally distinguished as systemic and local changes. For example, the origin of the body cavities evolved by several animal groups may have been initiated by simple changes in cell-to-cell adhesive properties. Any possible change in an existing ontogenetic pathway has the potential to generate novelties.